1. Field of the Invention
An improved method of treating a substrate material in an electrolyte containing fluoride ions wherein the substrate is anodically treated under controlled potential conditions to form an electrode.
2. Description of the Prior Art
It is known to produce electrolytic manganese dioxide from an electrolyte comprising a manganese sulfate-sulfuric acid solution. The manganese dioxide is deposited on the anode under carefully controlled conditions of electrolysis to produce a deposit having certain desired properties. The manganese dioxide so produced is recovered and used, for example, as a depolarizer for batteries.
When graphite is used as the anode in such an electrodeposition process, the current efficiency generally is limited to less than about 90%. Further, the manganese dioxide is strongly attached to the graphite surface and subsequent removal of the manganese dioxide also results in removal of particles of graphite, which necessarily contaminate the manganese dioxide. Moreover, it is difficult to remove the deposited manganese dioxide without damaging the graphite anode.
Certain other materials also have been suggested for use as anodes in the electrodeposition of manganese dioxide, such as, for example, platinum, platinum plated titanium and other noble metals and their oxides. The cost of such anodes is too high for an economical commercial operation. Titanium is substantially impervious to most corrosive environments (such as electrolytes), however, its susceptibility to passivation has limited its acceptance as a suitable anode material.
In U.S. Pat. 2,608,531 there is suggested a process for preparing electrolytic manganese dioxide using a particular type of titanium anode. Specifically, the patent discloses an anode consisting essentially of titanium particles compacted into a coherent but incompletely consolidated porous mass. However, such an anode is weak in mechanical characteristics and subject to breakage.
In U.S. Pat. 3,436,323 it is suggested that the disadvantages of a compacted titanium anode can be overcome by using an anode having a finely and sharply indented aventurine surface. Such surface preferably is obtained by sand blasting the surface of the anode. It has been found, however, when using such an anode, that an anodic current density in excess of about 9 amps per square foot results in passivation of the titanium anode surface.
Passivation is indicated by an increase in the electrical resistance of the anode. An increase in resistance necessarily requires an increase in the voltage applied to the anode to maintain a constant current density and also results in a corresponding increase in power consumption.
More recently, in U.S. Pat. 3,841,978 there is suggested a process for anodically treating a substrate material in an electrolyte comprising an aqueous solution containing fluoride ions and at least one compound selected from the group consisting of ethylene glycol, acetic acid and a mixture of phosphoric acid and nitric acid. The substrate is anodically treated at an anodic current density of at least 10 amps per square foot. During the treatment a gray film forms on the surface of the anode. The treatment time is a function of the current density applied during treatment. If the treatment time is of excessive duration, the film peels or flakes off the substrate material.
It is desirable to provide a method for treating a substrate material such that electrodes produced thereby will exhibit substantially similar behavioral characteristics during usage.